This invention relates to a cryogenic gas-fractionation process and apparatus, and in particular to an air fractionation system employing two-stage low-temperature rectification with a nitrogen liquefaction cycle. In said cycle, nitrogen is withdrawn in the gaseous phase from the head of the high-pressure stage and is liquefied by heating, compression, recooling, and expansion and then recycled to the high-pressure stage. For maintaining a small temperature difference at the cold end of the primary heat exchanger, a low-pressure expansion turbine is used to expand preheated compressed gas, e.g., slightly fractionated air withdrawn from the bottom of the high pressure column. For further details of such a system, reference is made to U.S. Pat. No. 3,285,028.
Conventional processes of this type have the serious drawback that large temperature differences occur between the streams to be heated and those to be cooled, due to their temperature-dependent heat capacities in the cold section of the liquefaction cycle. By virtue of these large temperature differences, during heat exchange, the entropy of the system is substantially increased resulting in a loss in exergonic property.